Adapt gtk_tree_view_get_path_at_pos() and
gtk_tree_view_get_drag_dest_row() to just return FALSE when bin_window
is NULL and not hit a warning.  Makes this case consistent with the tree
view not having a model.  Documentation has been updated to clarify
this, unit test has been added.

Fixes bug 539377, based on a patch by Bjorn Lindqvist.